You came up with this
example:
(defun make-inc (e)
(lambda (m) (+ e m)))
(let ((f (make-inc 3)))
(funcall f 5))
I agree that static scoping is much better in this example. But we
could fix this simply by imposing static scope on function arguments.
So, is there an equivalent scenario in which the proposed change to
`let' is clearly desireable?

The point is not whether `e' is being bound in a let or a defun. Rather, the
point is that we should be able to use a variable like `e' as a free variable
in a closure. We can do so only if it is governed by lexical binding.